Many types of DNA damage are known to induce recombination. Recent observations with other systems have indicated that there may be a direction to the recombination when nonreciprocal exchanges (gene conversion) occur. We have developed a system in which it is possible to examine whether damage in a chromosome can cause information to be preferentially transferred into or away from the chromosome. Yeast cells of one mating type are treated with the damaging agent and mated with untreated cells. The two cell types have mutations at different sites in the LYS2 gene. Prototrophs are selected and the recessive allele is determined. Using this system, we have found that ionizing radiation-induced gene conversion occurs in over 85% of the cases by the irradiated chromosome being the recipient of information from the undamaged chromosome. Associated with G-1 gene conversion events are reciprocal recombination events. These results are consistent with the model proposed by Resnick for the repair of DNA double-strand breaks; biochemical evidence suggests that these breaks are present at the time of cell mating.